The present invention relates to low viscosity, low base number sulfonates and to processes for the production of these sulfonates. The invention also relates to oil based compositions containing these low base number sulfonates.
Basic or neutral sulfonates may be used as additives in lubricating oils for petrol engines and diesel engines for example for vehicles or marine engines. Neutral and low base number sulfonates function primarily as detergents to keep engine surfaces clean. High base number sulfonates are primarily used to neutralize acids produced in the oil during use. These sulfonates may help to inhibit corrosion.
Neutral and low base number sulfonates for use as oil additives are usually prepared by the neutralization of a sulfonic acid with a basic salt such as a basic calcium salt e.g. calcium oxide or hydroxide in a suitable diluent oil. The sulfonate product may be a mixture of a number of species. In addition dispersed calcium hydroxide may be present.
The product of this process may display some basicity, for example if the basic salt is added in stoichiometric excess to that required for complete neutralization of the sulfonic acid, or some other basic component is present. The product in this case is said to be overbased.
The neutral metal salts of typical sulfonic acids are extremely viscous materials and would have a TBN, as measured by ASTM D-2896, of zero. Methods have been sought which permit lower viscosity products to be prepared. This has been achieved by the incorporation of chloride, formate and hydroxide ions into the product. The products of these techniques are not truly neutral but are slightly overbased in that they contain more base than that required to react stoichiometrically with the sulfonic acid.
U.S. Pat. No. 4,764,295 discloses a process for the production of low base number sulfonates from sulfonic acids which have alkyl radicals of C.sub.15 to C.sub.40 ; the process uses chloride containing salts and carboxylic acids such as formic acid. The products have relatively low viscosity but contain chloride.
High base number sulfonates are generally prepared by a process of neutralization with excess base (overbasing) followed by carbonation. Typically the sulfonic acid is neutralized with excess basic metal oxide or hydroxide in a suitable diluent. Some of the excess basic metal oxide or hydroxide is converted to metal carbonate via carbonation. Typically the reaction is carried out in the presence of hydrocarbon and/or polar solvents such as toluene/methanol and diluent oil; some or all of these solvents may be subsequently removed. The resulting product is a colloidal dispersion, in a diluent oil, of sub-micron particles of CaCO.sub.3 and Ca(OH).sub.2 which are sterically stabilised by the calcium sulfonate species produced by the reaction.
Sulfonates have been prepared from synthetic sulfonic acids which have in turn been prepared for example by the sulfonation of C.sub.12 to C.sub.60+ alkyl substituted benzene, or xylene or toluene compounds and mixtures thereof. It has been found that some synthetic sulfonic acids are difficult to neutralize with for example calcium hydroxide or lime to produce sulfonates which have acceptable properties; the attempted neutralization results in the production of gelatinous products which for example are solid at room temperature. This is a particular problem when trying to prepare Neutral or Low Base Number Sulfonates from such sulfonic acids. Methods have been proposed to overcome this problem associated with synthetic sulfonic acids. One such method for sulfonic acids of molecular weight 480-540 is described in GB 1 575 957 wherein a large stoichiometric excess, over that required for neutralization of the sulfonic acid, of calcium hydroxide is added to a portion of the sulfonic acid in a diluent to produce a reaction mixture; the remainder of the sulfonic acid is subsequently added to the mixture, this addition being less than that which would be required to fully react with the remaining calcium hydroxide in the mixture. In addition a solution containing a source of chloride ion is added to the mixture after the calcium hydroxide or lime addition. The chloride ion is believed to act as a fluidizer for the product formation and is beneficial in enabling the production of fluid, filterable products from certain sulfonic acids such as synthetic sulfonic acids. The addition of chloride promoter prevents the formation of gelatinous products; however, the final product, contains chloride.
The presence of chloride in calcium sulfonates and other metal sulfonates is a problem from a waste disposal and environmental point of view. When compositions containing such sulfonates are destroyed, e.g. by incineration, harmful chlorinated and polychlorinated biphenyls may be produced. Waste disposal of compositions based on chloride containing sulfonates is therefore a problem; it would be advantageous to be able to produce chloride-free low base number sulfonates especially those derived from high molecular weight sulfonic acids without a loss of the beneficial properties associated with the use of chloride in their manufacture.
Thus, a need exists for low base number sulfonates and methods for making such sulfonates which do not have the foregoing problems of high levels of chloride ions, high viscosity, and sediment.
U.S. Pat. No. 5,804,094, entitled "Low base number sulfonates" discloses low base number sulfonates derived from high molecular weight sulfonic acids that have low viscosity and are chloride free. In their preparation, neutralization of high molecular weight sulfonic acids or partially neutralized soaps of high molecular weight sulfonic acids is completed by the use of a high base number sulfonate in conjunction with a carboxylic acid.